Power amplifier and liquid jet printing apparatus

ABSTRACT

A power amplifier includes: a modulator pulse-modulating a drive waveform signal serving as a reference of a drive signal applied to an actuator and outputting a plurality of modulated signals; a digital power amplifier having a plurality of digital power amplifier stages each including a pair of push-pull switching elements, amplifying the power of the plurality of modulated signals, and outputting multi-value amplified digital signals; and a low pass filter smoothing the amplified digital signals and outputting the drive signal, wherein the modulator includes a control section switching one of a state where the same modulated signal is connected to two or more of the digital power amplifier stages and a state where different modulated signals are connected to different digital power amplifier stages to the other.

This application is a Continuation of U.S. application Ser. No.12/623,625, filed Nov. 23, 2009, which claims priority to JapanesePatent Application No. 2008-302744 filed on Nov. 27, 2008. The foregoingpatent applications are incorporated herein by reference.

BACKGROUND

1. Technical Field

The present invention relates to a power amplifier that amplifies thepower of a drive waveform signal serving as a reference for driving anactuator.

2. Related Art

In known liquid jet printing apparatuses, a liquid is ejected fromnozzles by applying a drive signal, the power of which is amplified by apower amplifier, to actuators such as piezoelectric devices. However,when the power of the drive signal is amplified by the use of an analogpower amplifier such as a push-pull transistor which is linearly driven,the power loss is great and a large heat sink for heat dissipation isnecessary.

JP-A-2005-329710 discloses a technique for reducing the power loss andmaking a heat sink unnecessary by amplifying the power of a drive signalby the use of a digital power amplifier.

As described in JP-A-2005-329710, when the power of the drive signal isamplified by the use of the digital power amplifier, it is necessary toremove a frequency component of a modulated signal with a low passfilter before amplifying the power. To satisfactorily remove thefrequency component of the modulated signals, a low pass filter having asteep frequency characteristic, that is, a high-order low pass filter,for stably transmitting a drive waveform signal component andsatisfactorily removing the modulated signal frequency component isnecessary. In this case, the potential difference between the terminalsof the coil used in the low pass filter increases, leading to a higherhysteresis loss.

SUMMARY

An advantage of some aspects of the invention is that it provides apower amplifier which can lower the order of a low pass filter andprovide a high-precision drive signal when the power of the drive signalis amplified by the use of a digital power amplifier.

According to an aspect of the present invention, a power amplifierincluding: a modulator that pulse-modulates a drive waveform signalserving as a reference of a drive signal applied to an actuator andoutputs a plurality of modulated signals, a plurality of digital poweramplifier stages each including a pair of push-pull switching elementsto amplify the power of the plurality of modulated signals and outputmulti-value amplified digital signals, a low pass filter that filtersthe amplified digital signals to output the drive signal, and a controlsection in the modulator that switches between a state where one of theplurality of modulated signals is connected to two or more of thedigital power amplifier stages and a state where the plurality ofmodulated signals are connected to different digital power amplifierstages.

According to this configuration, the modulated signals are amplified inthe digital power amplifier stages and combined into an amplifieddigital signal, thus the amplified digital signal is in pulses or insteps.

The number of steps of arrival potentials of the amplified digitalsignal indicates the number of potentials at which the amplified digitalsignal in pulses or in steps arrives.

According to this configuration, since the outputs of the plural digitalpower amplifier stages are combined into the amplified digital signals,the potential difference between the steps of arrival potentials of theamplified digital signals decreases, thereby lowering the order of thelow pass filter for removing a frequency component of the modulatedsignals from the amplified digital signals. It is also possible toacquire a high-precision drive signal by using multi-value signals asthe amplified digital signals.

By lowering the order of the low pass filter, it is also possible tosimplify the circuit configuration and to reduce the circuit size.

Since the potential difference between the steps of the arrivalpotentials of the amplified digital signals is small, it is possible tolower the withstanding voltage of the switching elements of the digitalpower amplifier, thereby reducing the circuit size.

Since the source potential can be lowered with the same currentconsumption, it is possible to reduce the circuit size and save thepower.

Particularly, when a front digital power amplifier stage is turned onand a rear digital power amplifier stage is turned off, it is possibleto regenerate the power and to further save the power.

By switching between the state where the same modulated signal isconnected to two or more of the digital power amplifier stages and thestate where the modulated signals are connected to the distinct digitalpower amplifier stages, it is possible to enhance the precision of thedrive signal.

In the power amplifier, the control section may connect the samemodulated signal to two or more of the digital power amplifier stages ina period when an input-output response characteristic of a pulsemodulation is nonlinear.

According to this configuration, it is possible to enhance the precisionof the drive signal to avoid the nonlinear part of the input-outputresponse characteristic of the pulse modulation.

In the power amplifier, the modulator may set a pulse modulationfrequency in a predetermined period to be higher than the pulsemodulation frequency in a period other than the predetermined period.

According to this configuration, it is possible to satisfactorily reducethe modulation frequency component of the pulse modulation by the use ofthe low pass filter.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be described with reference to the accompanyingdrawings, wherein like numbers reference like elements.

FIG. 1 is a front view schematically illustrating the configuration of aliquid jet printing apparatus employing a power amplifier according toan embodiment of the invention.

FIG. 2 is a plan view illustrating the vicinity of a liquid jet headused in the liquid jet printing apparatus shown in FIG. 1.

FIG. 3 is a block diagram illustrating a controller of the liquid jetprinting apparatus shown in FIG. 1.

FIG. 4 is a diagram illustrating a drive signal used to drive actuatorsof liquid jet heads.

FIG. 5 is a block diagram illustrating the configuration of a switchingcontroller.

FIG. 6 is a block diagram illustrating an example of a drive circuit foran actuator.

FIG. 7 is a block diagram illustrating a liquid jet head shown in FIG.6.

FIG. 8 is a block diagram illustrating a modulator shown in FIG. 6.

FIG. 9 is a block diagram illustrating a digital power amplifier shownin FIG. 6.

FIG. 10 is a block diagram illustrating the digital power amplifiershown in FIG. 9.

FIG. 11 is a block diagram illustrating a low pass filter shown in FIG.6.

FIG. 12 is a diagram illustrating an example of a triangular wavesignal.

FIG. 13 is a diagram illustrating a drive signal based on the triangularwave signal shown in FIG. 12.

FIGS. 14A and 14B are diagrams illustrating an input-output response ofpulse width modulation.

FIG. 15 is a diagram illustrating a drive signal based on a triangularwave signal shown in FIGS. 14A and 14B.

FIG. 16 is a diagram illustrating an input-output response of pulsewidth modulation carried out by the modulator shown in FIG. 18.

FIG. 17 is a diagram illustrating a triangular wave signal enabling thepulse width modulation shown in FIG. 16.

FIG. 18 is a diagram illustrating a drive signal based on the triangularwave signal shown in FIG. 17.

FIG. 19 is a diagram illustrating a drive signal based on the triangularwave signal shown in FIG. 17.

FIG. 20 is a diagram illustrating a frequency response characteristic ofthe low pass filter shown in FIG. 11.

FIG. 21 is a diagram illustrating a triangular wave signal used in themodulator shown in FIG. 8.

FIG. 22 is a diagram illustrating a drive signal based on the triangularwave signal shown in FIG. 19.

DESCRIPTION OF EXEMPLARY EMBODIMENTS

A power amplifier used in a liquid jet printing apparatus according toan embodiment of the invention will now be described. FIG. 1 is adiagram schematically illustrating the configuration of the liquid jetprinting apparatus according to an embodiment of the invention. FIG. 1shows a line-head printing apparatus in which a print medium 1 isconveyed in a direction of a left to right arrow and is subjected to aprinting operation in a print area in the course of conveyance thereof.

In FIG. 1, reference numeral 2 represents plural liquid jet headsdisposed above a conveyance line of the print medium 1. The liquid jetheads are arranged to form two lines in a print medium conveyingdirection and to extend in a direction intersecting the print mediumconveying direction and are fixed to a head fixing plate 11.

Plural nozzles are formed on the bottom surface of each liquid jet head2 and this surface is called a nozzle plane.

As shown in FIG. 2, the nozzles are arranged in a line shape in thedirection intersecting the print medium conveying direction for eachcolor of the liquid to be ejected, where the line is called a nozzleline and the line direction is called a nozzle line direction.

A line head covering the entire width in the direction intersecting theconveying direction of the print medium 1 is formed by the nozzle linesof all the liquid jet heads 2 arranged in the direction intersecting theprint medium conveying direction.

When the print medium 1 passes below the nozzle planes of the liquid jetheads 2, a liquid is ejected from plural nozzles formed in the nozzleplane to perform a printing operation.

The liquid jet heads 2 are supplied with four-color liquids of yellow(Y), magenta (M), cyan (C), and black (K) via liquid supply tubes fromliquid tanks (not shown).

By simultaneously ejecting a predetermined amount of liquid from thenozzles formed in each liquid jet head 2 to a predetermined position,minute dots are formed on the print medium 1.

By performing this operation for each color and once passing the printmedium 1 conveyed by a conveying section 4, a one-pass printingoperation can be carried out.

An electrostatic method, a piezoelectric method, or a film boilingliquid jet method can be employed as a method of ejecting a liquid fromthe nozzles of each liquid jet head 2. The piezoelectric method isemployed in this embodiment.

In the piezoelectric method, when a drive signal is applied to apiezoelectric device as an actuator, a vibration plate in a cavity isdeformed to cause a variation in pressure in the cavity and a liquid isejected from the nozzle due to the variation in pressure.

It is possible to adjust the amount of the ejected liquid by adjusting awave height of a drive signal or a voltage varying slope.

The invention may be similarly applied to other liquid jet method otherthan the piezoelectric method.

A conveying section 4 for conveying the print medium 1 in the conveyingdirection is disposed below the liquid jet heads 2.

In the conveying section 4, a conveying belt 6 is suspended on a drivingroller 8 and a driven roller 9 and an electric motor (not shown) isconnected to the driving roller 8.

An adsorption unit (not shown) for adsorbing the print medium 1 onto thesurface of the conveying belt 6 is disposed inside the conveying belt 6.

An air suction unit adsorbing the print medium 1 onto the conveying belt6 using a negative pressure or an electrostatic adsorption unitadsorbing the print medium 1 onto the conveying belt 6 with anelectrostatic force is used as the adsorption unit.

Accordingly, when a sheet of the print medium 1 is fed to the conveyingbelt 6 from a pickup section 3 by the use of a pickup roller 5 and thedriving roller 8 is rotationally driven by an electric motor, theconveying belt 6 rotates in the print medium conveying direction, andthe print medium 1 is adsorbed onto the conveying belt 6 by theadsorption unit and is conveyed.

A printing operation is performed by ejecting a liquid from the liquidjet heads 2 in the course of conveying the print medium 1.

The print medium 1 having been subjected to the printing operation isdischarged downstream to a sheet discharge section 10 in the conveyingdirection.

A print reference signal output device including a linear encoder isdisposed in the conveying belt 6.

The print reference signal output device monitors that the conveyingbelt 6 and the print medium 1 adsorbed to and conveyed by the conveyingbelt move in synchronization, outputs a pulse signal corresponding tothe necessary print resolution with the movement of the conveying belt 6after the print medium 1 passes through a predetermined position in aconveying path, and outputs a drive signal to the actuator 22 from adrive circuit to be described later on the basis of the pulse signal,whereby a predetermined color of liquid is ejected to a predeterminedposition on the print medium 1 and a predetermined image is drawn on theprint medium 1 by the use of dots.

A control device controlling the liquid jet printing apparatus isdisposed in the liquid jet printing apparatus according to thisembodiment.

As shown in FIG. 3, the control device includes an input interface 61reading print data input from a host computer 60, a control section 62including a micro computer and performing a computing operation such asa printing process on the basis of the print data input from the inputinterface 61, a pickup roller motor driver 63 controlling the driving ofa pickup roller motor 17 connected to a pickup roller 5, a head driver65 controlling the driving of the liquid jet heads 2, an electric motordriver 66 controlling the driving of an electric motor 7 connected tothe driving roller 8, and an interface 67 connecting the pickup rollermotor driver 63, the head driver 65, and the electric motor driver 66 tothe pickup roller motor 17, the liquid jet heads 2, and the electricmotor 7.

The control section 62 includes a CPU (Central Processing Unit) 62 aperforming various processes such as a printing process, a RAM (RandomAccess Memory) 62 c temporarily storing print data input via the inputinterface 61 and various data used to perform the printing process ofthe print data and the like or temporarily developing programs for theprinting process and the like, and a ROM (Read-Only Memory) 62 d formedof a nonvolatile semiconductor memory storing control programs executedby the CPU 62 a.

When the control section 62 receives the print data (image data) fromthe host computer 60 via the input interface 61, the CPU 62 a performs apredetermined process on the print data, calculates nozzle selectiondata (drive pulse selection data) such as data on what nozzle should jeta liquid or what amount of liquid should be ejected, and outputs a drivesignal and a control signal to the pickup roller motor driver 63, thehead driver 65, and the electric motor driver 66 on the basis of theprint data, the drive pulse selection data, and input data from varioussensors.

The pickup roller motor 17, the electric motor 7, and the actuators 22in the liquid jet heads 2 operate by the drive signal and the controlsignal, and processes of picking up, conveying, and discharging theprint medium 1 and the printing process of the print medium 1 areperformed.

The elements of the control section 62 are electrically connected toeach other via a bus (not shown).

FIG. 4 shows an example of a drive signal COM which is supplied to theliquid jet heads 2 from the controller of the liquid jet printingapparatus according to this embodiment and used to drive the actuators22 formed of piezoelectric devices.

In this embodiment, a signal of which the potential varies about amiddle potential is used.

The drive signal COM is obtained by connecting drive pulses PCOM as unitdrive signals for driving the actuators 22 to jet the liquid in timeseries. The rising edge of each drive pulse PCOM is a step where thevolume of a cavity (pressure chamber) communicating with the nozzle isenlarged to draw in the liquid (it may be considered that a meniscus isdrawn in, in consideration of the liquid ejecting plane), and thefalling edge of the drive pulse PCOM is a step where the volume of thecavity is reduced to press out the liquid (it may be considered that ameniscus is pressed out in consideration of the liquid ejecting plane).By pressing out the liquid, the liquid is ejected from the nozzle.

By variously changing the voltage variation slope or the wave height ofthe drive pulse PCOM having such a voltage-based waveform, the amount ofliquid drawn-in or the drawing-in speed and the amount of liquidpressed-out or the pressing-out speed can be changed. Accordingly, theamount of jet liquid can be changed to obtain dots having differentsizes.

Therefore, when plural drive pulses PCOM are connected in time series,it is possible to obtain dots having various sizes by selecting a singledrive pulse PCOM therefrom and supplying the selected drive pulse to theactuator 22 to jet the liquid, or by selecting plural drive pulses PCOMand supplying the selected drive pulses to the actuator 22 to jet theliquid plural times.

That is, when plural liquid droplets land at the same position with asthe liquid that has not dried, it is substantially the same as ejectinglarge liquid droplets, thereby enlarging the dots.

By combination of these techniques, it is possible to accomplish anincrease in gray scale.

The drive pulse PCOM1 at the left end of FIG. 4 serves to only draw inthe liquid but not to press out the liquid.

This is called minute vibration and is used to suppress or prevent thethickening of the nozzles without ejecting the liquid.

As the control signals from the controller shown in FIG. 3 in additionto the drive signal COM, the liquid jet head 2 is supplied with drivepulse selection data SI&SP for selecting the nozzles to eject the liquidon the basis of the print data and determining a connection time of thedrive signal COM to the actuators 22 of the piezoelectric devices, alatch signal LAT and a channel signal CH for connecting the drive signalCOM to the actuators 22 of the liquid jet heads 2 on the basis of thedrive pulse selection data SI&SP after nozzle selection data are inputinto all the nozzles, and a clock signal CLK for transmitting the drivepulse selection data SI&SP as a serial signal to the liquid jet heads 2.

In the following description, the minimum unit of drive signals fordriving the actuators 22 is referred to as drive pulse PCOM and theentire signal obtained by connecting the drive pulses PCOM in timeseries is referred to as drive signal COM.

That is, a series of drive signals COM is output as the latch signal LATand the drive pulse PCOM is output as each channel signal CH.

FIG. 5 shows the specific configuration of a switching controllerconstructed in the liquid jet heads 2 so as to supply the drive signalCOM (drive pulses PCOM) to the actuators 22.

The switching controller includes a shift register 211 storing the drivepulse selection data SI&SP for specifying the actuators 22 such as thepiezoelectric devices corresponding to the nozzles to jet the liquid, alatch circuit 212 temporarily storing data of the shift register 211,and a level shifter 213 connecting the drive signal COM to the actuators22 such as the piezoelectric devices by converting the level of theoutput of the latch circuit 212 and supplying the converted output tothe selection switch 201.

The shift register 211 is sequentially supplied with the drive pulseselection data SI&SP and a memory area is shifted from the initial stageto the subsequent stage on the basis of the input pulse of the clocksignal CLK.

The latch circuit 212 latches the output signals of the shift register211 on the basis of the input latch signal LAT after the drive pulseselection data SI&SP corresponding to the number of nozzles is stored inthe shift register 211.

The signals stored in the latch circuit 212 are converted into a voltagelevel which can turn on or off the selection switch 201 at thesubsequent stage by the level shifter 213.

This is because the drive signal COM has a voltage higher than theoutput voltage of the latch circuit 212 and thus the dynamic voltagerange of the selection switch 201 is set high.

Therefore, the actuators 22 such as the piezoelectric devices of whichthe selection switch 201 is turned off by the level shifter 213 areconnected to the drive signals COM (drive pulses PCOM) at the connectiontime of the drive pulse selection data SI&SP.

After the drive pulse selection data SI&SP of the shift register 211 isstored in the latch circuit 212, next print information is input to theshift register 211 and the data stored in the latch circuit 212 issequentially updated to correspond to the liquid ejecting time.

In the drawing, reference sign HGND represents a ground terminal of theactuators 22 such as the piezoelectric devices.

Even after the actuators 22 such as the piezoelectric devices aredisconnected from the drive signal COM (drive pulses PCOM), the inputvoltage of the corresponding actuators 22 is maintained in the voltagejust before the disconnection by the selection switch 201.

FIG. 6 shows the schematic configuration of the drive circuit of theactuators 22.

Plural nozzles are formed in the liquid jet heads 2 of the line-headprinting apparatus, the above-mentioned actuator 22 is disposed in eachnozzle as shown in FIG. 7, the selection switch 201 formed of atransmission gate is disposed on the upstream side of the actuator 22,and the drive signal COM (drive pulses PCOM) are applied to only theactuator 22 of which the selection switch 201 is turned on.

The drive circuit includes a drive waveform generator 25 generating thesource of the drive signal COM (drive pulses PCOM), that is, a drivewaveform signal WCOM as a reference of a signal controlling the drivingof the actuator 22, on the basis of previously stored waveform data, amodulator 26 pulse-modulating the drive waveform signal WCOM generatedby the drive waveform generator 25, a digital power amplifier 28amplifying the power of the modulated signal pulse-modulated by themodulator 26, and a low pass filter 29 smoothing the amplified digitalsignal of which the power has been amplified by the digital poweramplifier 28 and supplying the smoothed amplified digital signal as thedrive signal COM (drive pulses PCOM) to the actuators 22 of the liquidjet heads 2.

The drive waveform generator 25 combines and outputs the predetermineddigital potential data in time series, converts the combined digitalpotential data into analog data by the use of a D/A converter, andoutputs the converted data as the drive waveform signal WCOM.

In this embodiment, a pulse width modulator (PWM) is used as themodulator 26 pulse-modulating the drive waveform signal WCOM.

As known in the past, the pulse width modulator generates a referencesignal such as a triangular wave signal or a saw-toothed signal of apredetermined frequency, compares the drive waveform signal WCOM withthe reference signal, and outputs as the modulated signal a pulse signalwhich is on-duty when the drive waveform signal WCOM is higher than thereference signal.

However, normally in this embodiment, as shown in FIG. 8, a triangularwave signal which is generated from a triangular wave generator 33 andof which the wave height is equal to about a source potential VHV isinput to the first comparator 34 a, the comparison result with the drivewaveform signal WCOM is output as a first modulated signal PWM1, asignal obtained by adding the source potential VHV to the triangularwave signal is input to a second comparator 34 b, and the comparisonresult with the drive waveform signal WCOM is output as a secondmodulated signal PWM2.

When the selection switch 36 is switched by the control section 35, thedouble value of the triangular wave signal generated from the triangularwave signal generator 33 is input to the first and second comparators 34a and 34 b, and the comparison results with the drive signal waveformsignal WCOM is output as the first and second modulated signals PWM1 andPWM2.

That is, when the selection switch 36 is switched by the control section35, the first modulated signal PWM1 and the second modulated signal PWM2have the same value and thus the power amplifier to be described lateris synchronized.

The triangular wave signal generator 33 can change the frequency of thetriangular wave signal and the control section 35 changes the frequencyof the triangular wave signal to a frequency higher than the normal caseat the time of switching the selection switch 36.

The details thereof will be described later.

In this embodiment, since two digital power amplifier stages aredisposed in the digital power amplifier 28, the modulator 26 outputs themodulated signals corresponding to plural digital power amplifierstages.

As shown in FIG. 9, the digital power amplifier 28 includes a firstdigital power amplifier 27 a amplifying the power of the first modulatedsignal PWM1 and a second digital power amplifier 27 b amplifying thepower of the second modulated signal PWM2.

The high side of the first digital power amplifier 27 a is connected tothe power source VHV and the low side is grounded.

A bootstrap circuit 32 is interposed between the first digital poweramplifier 27 a and the second digital power amplifier 27 b. The highside of the second digital power amplifier 27 b is connected to thepower source VHV via a commutator D of the bootstrap circuit 32 and thelow side thereof is connected to the output terminal of the firstdigital power amplifier 27 a.

That is, the low side of the second digital power amplifier 27 bcorresponding to the rear stage is biased by the output of the firstdigital power amplifier 27 a corresponding to the front stage.

The bootstrap circuit 32 includes a commutator D regulating the currentfrom the high side of the second digital power amplifier 27 b and acapacitor CB charged with the potential difference between the powersource VHV and the output of the first digital power amplifier 27 a.

The capacity of the capacitor CB is set to be enough to drive theactuator 22 which is a capacitive load formed of a piezoelectric device.

Specifically, the capacity is set to a capacity capable of guaranteeinga bootstrap potential when the first digital power amplifier 27 a of thefront stage is turned on and the second digital power amplifier 27 b ofthe rear stage is turned off.

As shown in FIG. 10, the first and second digital power amplifiers 27 aand 27 b each include a half bridge output terminal 31 including ahigh-side switching element Q1 and a low-side switching element Q2 forsubstantially amplifying the power and a gate driver circuit 30 foradjusting gate-source signals GH and GL of the high-side switchingelement Q1 and the low-side switching element Q2 on the basis of themodulated signals from the modulator 26.

The gate-source signals GH and GL of the high-side switching element Q1and the low-side switching element Q2 are inverted signals.

In the digital power amplifiers 27 a and 27 b, when the modulated signalis at a high level, the gate-source signal GH of the high-side switchingelement Q1 is at the high level and the gate-source signal GL of thelow-side switching element Q2 is at a low level. Accordingly, thehigh-side switching element Q1 is switched to the ON state and thelow-side switching element Q2 is switched to the OFF state. As a result,the output of the half bridge output terminal 31 has a high-sidepotential.

On the other hand, when the modulated signal is at a low level, thegate-source signal GH of the high-side switching element Q1 is at thelow level and the gate-source signal GL of the low-side switchingelement Q2 is at the high level. Accordingly, the high-side switchingelement Q1 is switched to the OFF state and the low-side switchingelement Q2 is switched to the ON state. As a result, the output of thehalf bridge output terminal 31 has a low-side potential.

In this way, when the high-side and low-side switching elements aredriven in a digital manner, current flows in the switching element inthe ON state, but the resistance value between the drain and the sourceis very small and thus the power loss is hardly caused.

Since no current flows in the switching element in the OFF state, thepower loss is not caused.

Therefore, the power loss of the digital power amplifiers 27 a and 27 bis very small and thus a switching element such as a small-sized MOSFETcan be employed, thereby making a cooling mechanism such as a coolingheat sink unnecessary.

In addition, the efficiency is about 30% when a transistor is linearlydriven, but the efficiency of the digital power amplifier is 90% ormore.

The cooling heat sink of the transistor needs to have a size of 60 mmsquare for each transistor. Accordingly, when the cooling heat sink ismade unnecessary, it is very advantageous in view of an actual layout.

As shown in FIG. 11, the low pass filter 29 includes a combination of acoil L and a capacitor C and removes the modulation frequency componentof the amplified digital signal APWM, that is, the frequency componentof the triangular wave signal.

In this embodiment, the first digital power amplifier 27 a is disposedas the rear stage of the second digital power amplifier 27 b of thefront stage of which the high side is connected to the power source VHVand the low side of the first digital power amplifier 27 a isbootstrapped up to the potential of the power source VHV by thebootstrap circuit 32. Accordingly, when the first digital poweramplifier 27 a is turned off, the output of the second digital poweramplifier 27 b is output as the amplified digital signal APWM from thefirst digital power amplifier 27 a. However, when the first digitalpower amplifier 27 a is turned on, the added value of the output of thefirst digital power amplifier 27 a and the output of the second digitalpower amplifier 27 b is output as the amplified digital signal APWM fromthe first digital power amplifier 27 a.

The operation of the control section 35 of the modulator 26 will now bedescribed.

As described above, at the normal time when the selection switch 36 isnot switched by the control section 35, the triangular wave signal whichis generated from the triangular wave signal generator 33 and of whichthe wave height is equal to about the source potential VHV is input tothe first comparator 34 a as shown in FIG. 12 and a signal obtained byadding the source potential VHV to the triangular wave signal is inputto the second comparator 34 b.

Since the voltages of the two triangular wave signals do not overlapwith each other, the first modulated signal PWM1 of the first and secondmodulated signals PWM1 and PWM2 as the comparison result of thetriangular wave signals and the drive waveform signal WCOM holds thehigh level in the area where the drive waveform signal WCOM (the drivesignal COM in the drawing) is equal to or higher than the sourcepotential VHV as shown in FIG. 13.

The amplified digital signal APWM has a value obtained by adding thefirst modulated signal PWM1 and the second modulated signal PWM2, theamplified digital signal APWM of the first modulated signal PWM1amplified by the first digital power amplifier 27 a has a pulse betweenthe source potential VHV and the 0 potential, the amplified digitalsignal APWM of the second modulated signal PWM2 amplified by the seconddigital power amplifier 27 b and added thereto has a pulse between thesource potential VHV and the double value thereof VHV×2.

Therefore, since the amplified digital signal APWM having the addedvalue of two amplified digital signals has the 0 potential, the sourcepotential VHV, and the double value of the source potential VHV×2 asarrival potentials, the number of steps of arrival potentials is “3” andis greater than the number of stages “2” of the digital power amplifiers27 a and 27 b.

As the number of steps of arrival potentials of the amplified digitalsignal APWM before being smoothed by the low pass filter 29 increases,the waveform precision of the drive signal COM (drive pulses PCOM) afterbeing smoothed, improves.

The source potential VHV is a half the wave height of the drive signalCOM, that is, the amplified digital signal APWM, which is excellent.Accordingly, it is possible to satisfactorily remove the modulationfrequency even when the frequency characteristic of the low pass filter29 is relatively slow.

In other words, since the order of the low pass filter 29 can belowered, the size can be reduced, and the potential difference betweenterminals of the coil L can be reduced, the loss due to the hysteresiscan be reduced.

Since the total current flowing in the two-stage digital poweramplifiers 27 a and 27 b is constant but the source potential VHV isabout a half of the wave height of the drive signal COM, that is, theamplified digital signal APWM, which is excellent, it is possible tosave the power and to lower the withstanding voltage of the switchingelements Q1 and Q2 of the digital power amplifiers 27 a and 27 b,thereby reducing the circuit size.

When the first digital power amplifier 27 a of the front stage is turnedon and the second digital power amplifier 27 b of the rear stage isturned off by the bootstrap circuit 32, the power regeneration that theelectric charges of the actuators 22 as the capacitive load and thecapacitor CB of the bootstrap circuit 32 flow to the power source VHV iscaused, thereby further saving the power.

However, since the on-duty or off-duty pulse width of the input-outputresponse characteristic of pulse width modulation is small and thus theresponse is not sufficient, a nonlinear area exists in the vicinity of0% or 100% of the pulse duty ratio (duty ratio in the drawing) as shownin FIG. 14A. As shown in FIG. 12, when each of the first modulatedsignal PWM1 and the second modulated signal PWM2 uses 0% to 100% of thepulse duty ratio without overlapping two triangular wave sinals, thenonlinear area appears in the switching parts of the first modulatedsignal PWM1 and the second modulated signal PWM2 as shown in FIG. 14B.(Similarly, the vicinities of the output voltage 0 V and VHV×2V whichare nonlinear may not be used by setting the duty ratio.)

The switching parts of the first modulated signal PWM1 and the secondmodulated signal PWM2 are also the switching parts of the first digitalpower amplifier 27 a of the front stage and the second digital poweramplifier 27 b of the rear stage and are also the switching time of thearrival potentials of amplified digital signal APWM, that is, the sourcepotential VHV, and the vicinity thereof.

As a result, as shown in FIG. 15, a distortion occurs in the amplifieddigital signal APWM, whereby the drive signal COM (drive pulses PCOM) isalso distorted.

Therefore, in this embodiment, as shown in FIG. 16, in the switchingtime of the arrival potentials of the amplified digital signal APWM ofwhich the input-output response characteristic of pulse width modulationis nonlinear and a predetermined period including the switching time,the first digital power amplifier 27 a and the second digital poweramplifier 27 b are synchronized for the switching and the same amplifieddigital signal APWMSYNC is output from the two digital power amplifiers27 a and 27 b.

Specifically, as shown in FIG. 16, in the switching time of the firstmodulated signals PWM1 and the second modulated signal PWM2 and apredetermined period including the switching time, that is, when thedrive waveform signal WCOM is in a predetermined area, the selectionswitch 36 is switched by the control section 35 and the triangular wavesignal of which the wave height is double is input to both of the firstcomparator 34 a and the second comparator 34 b.

When the triangular wave signal of which the wave height is double isinput to both of the first comparator 34 a and the second comparator 34b, the first modulated signal PWM1 and the second modulated signal PWM2output from the two comparators 34 a and 34 b have the same value, thatis, become the synchronized modulated signal PWMSYNC. That is, thesynchronized voltage-amplified modulated signal APWMSYNC is output fromthe two digital power amplifiers 27 a and 27 b synchronized with eachother.

That is, the control section 35 makes a control to switch one of thestates where the same modulated signal APWMSYNC is connected to twodigital power amplifiers 27 a and 27 b and the state where differentmodulated signals PWM1 and PWM2 are connected to different digital poweramplifiers 27 a and 27 b, respectively, to the other.

In this way, by synchronizing the first digital power amplifier 27 a ofthe front stage and the second digital power amplifier 27 b of the rearstage with each other for the switching and outputting the sameamplified digital signal APWMSYNC from the two digital power amplifiers27 a and 27 b in the switching time of the arrival potentials of theamplified digital signal APWM of which the input-output responsecharacteristic of pulse width modulation is nonlinear and apredetermined period including the switching time, it is possible toavoid the nonlinear area of the input-output response characteristic ofthe first modulated signal PWM1 and the second modulated signal PWM2 andthus to enhance the waveform precision of the drive signal COM (drivepulses PCOM).

FIG. 18 shows the amplified digital signal APWM and the drive signal COM(drive pulse PCOM) of which the distortion can be removed by the use ofthe triangular wave signal shown in FIG. 17.

However, a case can be considered where the distortion can be removed bythe use of the triangular wave signal shown in FIG. 17 but thesatisfactory waveform precision of the drive signal COM (drive pulsesPCOM) cannot be obtained.

An example thereof is shown in FIG. 19.

This is an example where the frequency component of the triangular wavesignal, that is, the modulation frequency component, remains in thedrive signal COM (drive pulses PCOM) because the first modulated signalPWM1 and the second modulated signal PWM2 are the same synchronizedmodulated signal PWMSYNC and the voltage amplitude is doubled. In thiscase, the modulation frequency component cannot be satisfactorilyreduced by the low pass filter 29.

To satisfactorily reduce the remaining modulation frequency component bythe use of the low pass filter 29, the frequency characteristic of thelow-pass filter should be steep, that is, the gain of the modulationfrequency domain should be made very small. Accordingly, as describedabove, the order of the low-pass filter should be raised and theoriginal problem is caused again.

Therefore, in this embodiment, as shown in FIG. 20, the frequenciesshould be enhanced at the time of synchronizing the first modulatedsignal PWM1 and the second modulated signal PWM2, so as tosatisfactorily reduce the modulation frequency component fPWMSYNC withlow-order low-pass filters having the same frequency characteristic.

Specifically, the control section 35 controlling the switching of theselection switch 36 controls the operating frequency of the triangularwave signal generator 33 to raise the frequency of the triangular wavesignal.

FIG. 21 shows a state where the selection switch 36 is switched by thecontrol section 35 and the operating frequency of the triangular wavesignal generator 33 is controlled to input the triangular wave signalwith a double wave height and a high frequency to both of the firstcomparator 34 a and the second comparator 34 b in the switching time ofthe first modulated signal PWM1 and the second modulated signal PWM2 anda predetermined period including the switching time, that is, when thedrive waveform signal WCOM is in a predetermined area.

FIG. 22 shows the drive signal COM (drive pulses PCOM) when the firstdigital power amplifier 27 a of the front stage and the second digitalpower amplifier 27 b of the rear stage are synchronized and themodulation frequency is raised in the nonlinear periods of the firstmodulated signal PWM1 and the second modulated signal PWM2.

The modulation frequency component is satisfactorily reduced from thedrive signal COM (drive pulses PCOM), thereby obtaining high waveformprecision.

As described above, in the power amplifier according to this embodiment,the drive waveform signal WCOM serving as a reference for driving theactuators 22 is pulse-modulated into two (plural) modulated signals PWM1and PWM2, the pulse-modulated two (plural) modulated signals PWM1 andPWM2 are amplified in power into the multi-value amplified digitalsignal APWM, and the amplified digital signal APWM is smoothed andoutput to the actuators 22. Accordingly, by combining the outputs ofplural-stage digital power amplifiers 27 a and 27 b into the amplifieddigital signal APWM, the potential difference between the steps of thearrival potentials of the amplified digital signal APWM can be reducedand the order of the low pass filter 29 removing the frequency componentof the modulated signal from the amplified digital signal APWM can belowered. In addition, by using the multi-value signal as the amplifieddigital signal APWM, it is possible to obtain the drive signal COM(drive pulses PCOM) with high precision.

By lowering the order of the low pass filter 29, it is possible tosimplify the circuit configuration and to reduce the circuit size.

Since the potential difference between the steps of the arrivalpotentials of the amplified digital signal APWM is small, thewithstanding voltage of the switching elements Q1 and Q2 of the digitalpower amplifiers 27 a and 27 b can be lowered, thereby reducing thecircuit size.

The source potential VHV can be lowered even with the same currentconsumption, thereby reducing the circuit size and saving the power.

Particularly, when the digital power amplifier 27 a of the front stageis turned on and the digital power amplifier 27 b of the rear stage isturned off, the power regeneration is caused, thereby further saving thepower.

By allowing the control section 35 to switch one of the states where thesame modulated signal is connected to two digital power amplifiers 27 aand 27 b and the state where different modulated signals are connectedto different digital power amplifiers 27 a and 27 b, respectively, tothe other, it is possible to enhance the precision of the drive signalCOM (drive pulses PCOM).

By connecting the same modulated signal to the two digital poweramplifiers 27 a and 27 b in the period when the input-output responsecharacteristic of pulse modulation is nonlinear, it is possible to avoidthe nonlinear part of the input-output response characteristic of pulsemodulation and to enhance the precision of the drive signal COM (drivepulses PCOM).

By setting the pulse modulation frequency in a predetermined period tobe higher than the pulse modulation frequency in a period other than thepredetermined period, it is possible to satisfactorily reduce themodulation frequency component of pulse modulation by the use of the lowpass filter 29.

In this embodiment, two stages of digital power amplifiers 27 a and 27 bare provided, but the number of digital power amplifier stages may bethree or more. When a bootstrap circuit is provided in the digital poweramplifier of the rear stage and the voltage is biased by the digitalpower amplifier of the front stage, it is possible to obtain a highoutput voltage with a small source potential.

The bootstrap circuit is not essential, and any circuit can be used aslong as the power of plural modulated signals is amplified by pluraldigital power amplifiers.

A pulse density modulation (PDM) circuit may be used instead of thepulse width modulation circuit as the modulator. In this case, it ispossible to obtain the drive signal COM (drive pulses PCOM) with higherprecision.

In the above-mentioned embodiment, since a power source is shared by theplural-stages of digital power amplifiers by connecting the plural-stagedigital power amplifiers to the same power source, the circuit size canbe reduced. However, the plural-stage digital power amplifiers may beconnected to power sources having different potentials. In this case, itis possible to enhance the number of multi values of the amplifieddigital signal, thereby obtaining the drive signal with higherprecision.

In the above-mentioned embodiment, the power amplifier is used in theline-head liquid jet printing apparatus. However, the power amplifieraccording to the embodiment of the invention may be used in a multi-passliquid jet printing apparatus.

In the above-mentioned embodiment, the power amplifier is applied to theliquid jet printing apparatus, but the invention is not limited to theembodiment. The power amplifier may be applied to a liquid ejectingapparatus jetting or ejecting liquids other than ink (including liquidmaterials in which functional material particles are dispersed and fluidmaterials such as gel) or fluids other than the liquids (such as solidmaterials which can be ejected as fluid).

Examples of the liquid ejecting apparatus include a liquid materialejecting apparatus ejecting liquid materials in which electrodematerials or coloring materials used to manufacture a liquid crystaldisplay, an EL (Electroluminescence) display, a surface emissiondisplay, a color filter, and the like are dispersed or dissolved, aliquid ejecting apparatus ejecting biological organics used tomanufacture a biological chip, and a liquid ejecting apparatus ejectinga liquid as a sample and being used as a precise pipette.

The examples of the liquid ejecting apparatus may include a liquidejecting apparatus ejecting lubricant to a precise machine such as awatch or a camera by the use of a pin point, a liquid ejecting apparatusejecting onto a substrate a transparent resin liquid such as UV-curableresin to form micro semi-spherical lenses (optical lenses) used inoptical communication devices, a liquid ejecting apparatus ejectingetchant such as acid or alkali to etch a substrate or the like, a fluidmaterial ejecting apparatus ejecting gel, and a fluid-ejecting recordingapparatus ejecting solid powder materials such as toner.

The invention can be applied to any of the above-mentioned ejectingapparatuses.

What is claimed is:
 1. A power amplifier comprising: a modulator thatpulse-modulates a drive waveform signal serving as a reference of adrive signal applied to an actuator to output at least twopost-modulated signals; a digital power amplifier that amplifies the atleast two post-modulated signals to output multi-value amplified digitalsignals, the digital power amplifier having a plurality of digital poweramplifier stages each including a pair of push-pull switching elements;and a low pass filter that filters the amplified digital signals andoutputs the drive signal, wherein the modulator includes a controlsection switching between three states of modulations according to time,including: a first state where the modulator outputs a firstpost-modulated signal of the at least two post-modulated signals to thedigital power amplifier, and the modulator outputs a secondpost-modulated signal, low to the digital power amplifier, a secondstate where the modulator outputs the second post-modulated signal ofthe at least two post-modulated signals, high to the digital poweramplifier, and a third state where the modulator outputs the samepost-modulated signal from the first post-modulated signal and thepost-second modulated signal to the digital power amplifier.
 2. Thepower amplifier according to claim 1, wherein the control sectionconnects one of the modulated signals to two or more of the digitalpower amplifier stages in a nonlinear period, that is when aninput-output response characteristic of pulse modulation is nonlinear.3. The power amplifier according to claim 2, wherein the modulator setsa pulse modulation frequency higher in the nonlinear period than a setfrequency out of the nonlinear period.
 4. A liquid jet printingapparatus employing the power amplifier according to claim 1.